IMP 8 observations of traveling compression regions in the mid-tail near substorm expansion phase onset

Abstract

The large data set returned by the IMP 8 magnetic field investigation has been examined to understand the characteristics of traveling compression regions (TCRs). Using 15 years of the AL index, we identified 565 isolated substorm events with well-developed expansion phase for which IMP 8 was in the mid-tail lobes and providing magnetic field measurements. From this data set, 17 substorms were found to produce the bipolar BZ TCRs frequently observed farther down the tail. However, another 14 cases have field compressions during which the field tilts north-then-south, but the maximum BZ does not reach a positive value, i.e., the BZ variation is not bipolar. These "negative BZ" compression regions (NCRs) are examined in some detail and found to occur at larger IZI values. The usual bipolar BZTCRs, in contrast, are found at smaller |Z| values closer to lobe-plasma sheet interface. We interpret an NCR as a TCR which is observed at large |Z| where the growth phase flaring is too strong for the lobe field draping about the plasmoid to produce a positive BZ between the leading edge and the center of the compression region. Hence, at IMP 8 distances TCRs are often not accompanied by true bipolar BZ signatures; rather only a north-then-south tilting of the field relative to flared lobe field is observed.